The optical grating described here, more precisely the periodic structure thereof, typically has a so-called blaze structure in order to facilitate the diffraction of the incident radiation into a predetermined order of diffraction. A blaze structure or a blaze grating typically has a stepped embodiment and a respective structure has a substantially triangular cross section in the style of a sawtooth. The periodic structure in the form of the blaze structure is optimized in such a way that as much intensity of the incident radiation as possible is concentrated in a predetermined order of diffraction (typically in the 1st order of diffraction), while the intensity that is diffracted into the other orders of diffraction is ideally minimized. Since, typically, this can only be achieved exactly for a single predetermined wavelength, a blaze grating is characterized not only by the order of diffraction, for which the optical grating is optimized, but also by the predetermined wavelength (laser wavelength), which is diffracted into this order of diffraction.
By way of example, optical gratings with blaze structures can be used as monochromators for EUV radiation, i.e., for radiation at wavelengths between approximately 5 nm and approximately 35 nm. In this wavelength range, the optical grating is typically embodied as a reflective grating, which serves to select the predetermined wavelength from the incident radiation, i.e., the radiation reflected or diffracted at the optical grating ideally only has the predetermined wavelength, i.e., the latter is monochromatized. However, it is not possible, as a rule, to suppress all secondary orders in such optical gratings, and so there are unwanted radiation components at other wavelengths in the monochromatized EUV radiation, which can be traced back to the diffraction into higher orders of diffraction.
A further problem is presented by the polarizing effect during the reflection or diffraction at such an optical grating since the optical grating typically has different reflection properties for a polarization component perpendicular to a plane of incidence of the incident radiation (s-polarization) than for a polarization component parallel to a plane of incidence of the incident radiation (p-polarization). Even in the case of a possible total internal reflection, there is a polarization of the radiation reflected or diffracted at the optical grating.
DE 10 2015 203 572 A1 has disclosed an optical grating that has a periodic structure, which is formed by a superposition of a blaze structure and a periodic modification structure. The modification structure can have the same period as the blaze structure and therefore have a fixed phase relation to the blaze structure. This should bring about a suppression of the higher orders of diffraction in the light reflected by the blaze structure.